1. Field Of The Invention
This invention relates to pneumatic tires, and more particularly to tread reinforcements or reinforcement belts for radial ply tires.
2. Prior Art
It is known in the tire industry to provide pneumatic tire constructions which include an annular reinforcement belt or breaker assembly about the periphery of the tire between the tread and the carcass. It has been found that such a pneumatic tire construction enhances the road stability of the tire and increases the tread life in both radial and bias ply constructions. Usually, the reinforcement belt consists of one or more annular bands which may be positioned parallel and concentrically adjacent to one another. These bands almost universally contain cords of organic, steel or glass materials which may be oriented on a bias angle from the equatorial plane of the tire. A major problem attending such reinforcement belt constructions is that of belt edge separation which generally arises from road stresses acting in the shoulder regions of the tire where the edges of the belts lie. These stresses are caused primarily by the rotation of the load-bearing tire into its contact patch with the pavement. It is the continual change in shape of the cross-section of the tire that causes the stresses, concentrating them in the shoulder regions. Since the belt edges within the shoulder region usually contain exposed ends of the bias oriented inextensible cords, the interface between the cords and the surrounding rubber is often the weakest bond area between the belts and the tire carcass. The shoulder region stresses have a tendency to cause a separation of the cord ends from the adjacent rubber, therefore resulting in a gradual ply separation.
Several variations in reinforcement belt constructions have been designed in an attempt to overcome the above difficulties. Specifically, there is disclosed in U.S. Pat. No. 3,881,538 a multi-ply reinforcing breaker consisting of a plurality of superimposed plies which decrease successively in span axially of the tire from the radially innermost ply to the radially outermost ply. The innermost ply is folded at its end to in part overlap at least one of the other plies to resist edge separation of the plies. Other reinforcement belts containing folded plies are described in U.S. Pat. Nos. 3,757,844, 3,949,797 and Canadian Pat. No. 952,419.
Usually, the various plies consist of steel cables or cords encased in rubber. The use of steel cables in the plies which give the belt increased structural rigidity leads to a harsher riding tire. It has been proposed to use materials consisting of aromatic polyamides, for instance those referred to as Aramides which have a modulus of elasticity preferably in excess of 3,500 kg/sq. mm or 260 gram/denier. Indeed, this material has attractive qualities of elasticity, toughness, low elongation, low specific mass, etc., under such conditions that it reduces the tendancy to separate or tear off the belt plies, specifically in the areas of the lateral edges of the plies, at least as long as an excessive speed has not been attained.
Construction for reinforcement belts including one or more non-metallic cord plies are detailed in U.S. Pat. Nos. 3,404,721, 3,690,364, 3,881,538, 3,949,797 and French Pat. No. 1,228,241. In several instances in which non-metallic cords are used, the softer riding characteristics and lighter weight attributable to the non-metallic cords are outweighed by the reduced structural integrity of the reinforcement belt structure.
Furthermore, known approaches to reinforcement belt designs do not provide completely satisfactory solutions to the problems of tire stiffness and shoulder fatigue.
It is toward the elimination of these and other drawbacks that the present invention is directed.